1. Field of the Invention
The present invention relates to a fluid dynamic pressure bearing device, a spindle motor provided with the bearing device and a disk drive apparatus provided with the spindle motor. The present invention also relates to a bearing mechanism using fluid dynamic pressure, a spindle motor, and a disk drive apparatus.
2. Description of the Related Art
In recent years, a storage disk drive apparatus has been used in a personal computer, a car navigation and so forth. The storage disk drive apparatus is required to have increased density while also being small-sized, low-profile and lightweight. Demands for a high rotation number and a highly accurate rotational operation exist in a spindle motor used in rotating disks.
A conventional fluid dynamic pressure bearing device includes a conical dynamic pressure bearing unit for radially and axially supporting a shaft or a sleeve. As the shaft and the sleeve rotation relative to one another, a fluid dynamic pressure is generated in the lubricating fluid filled in a minute gap by the pumping action of dynamic pressure groove arrays of the conical dynamic pressure bearing unit. The shaft or the sleeve is radially and axially supported by the fluid dynamic pressure thus generated.
However, with the conventional dynamic pressure bearing device it is sometimes the case that a strong impact caused by external factors is applied to the fluid dynamic pressure bearing device in a tapering seal portion formed between the outer circumferential surface of an annular member and the inner circumferential surface of a seal member (or the inner circumferential surface of a rotating member such as a hub or the like in case of not employing the seal member). At this time, the width of a minute gap between the radial outermost portion of the annular member in a cross-section containing a center axis and the inner circumferential surface of the seal member (or the inner circumferential surface of the rotating member) becomes momentarily zero. As a result, the annular member and the seal member (or the rotating member) make contact with each other in the zero-width region. The lubricating fluid held in the tapering seal portion then momentarily leaks out from the zero-width region.
Some conventional electric motors include a bearing mechanism using fluid dynamic pressure. For example, a fluid dynamic bearing apparatus used in a spindle motor disclosed in JP-A 2007-162759 includes a shaft and a tubular sleeve body inside which the shaft is inserted. The shaft is fixed to a base plate of the motor. The sleeve body is fixed to a rotor of the motor. The shaft is provided with two annular thrust flanges which are arranged above and below the sleeve body, respectively. The fluid dynamic bearing apparatus includes a radial bearing portion, which is arranged between the shaft and the sleeve body, and thrust bearing portions, which are arranged between each of the two thrust flanges and the sleeve body. As a result, the sleeve body and the rotor are rotatably supported relative to the shaft. The sleeve body has a communicating hole defined therein so as to communicate two thrust gaps with each other. Interfaces for lubricating oil are formed in the vicinity of upper and lower end openings of the communicating hole.
A fluid dynamic bearing motor disclosed in JP-A 2000-245104 includes a shaft fixed to a base, and a sleeve arranged to rotate around the shaft. A disc-shaped thrust plate made of stainless steel is fixed to the shaft. The sleeve is provided with an annular thrust bushing made of a different type of stainless steel. The thrust plate and the thrust bushing are arranged opposite to each other along a direction parallel to the shaft. The thrust plate and the thrust bushing together define a thrust gap therebetween. The thrust bushing in this fluid dynamic bearing motor is made of a stainless steel of superior durability, and this contributes to preventing an edge of the thrust plate from damaging the thrust bushing.
However, in the case of a bearing mechanism having the structure as described in JP-A 2007-162759, it is difficult to discharge air bubbles generated within the lubricating oil through an interface of the lubricating oil during the drive of the motor.
Some conventional electric motors include a bearing mechanism using fluid dynamic pressure. For example, in the spindle motor disclosed in FIG. 3 of JP-A 2005-48890, a hub 6 provided in a rotor is rotatably supported with a shaft 3 inserted therein, as described in paragraph 0034 of JP-A 2005-48890. As described in paragraph 0038 of JP-A 2005-48890, after the shaft 3 is inserted inside the hub 6, a seal plate 14 is press fitted to a top portion of the shaft 3. The seal plate 14 is arranged in close proximity to an upper end surface of an inner sleeve 9 of the hub 6. As described in paragraph 0039 of JP-A 2005-48890, the upper end surface of the inner sleeve 9 includes dynamic pressure generating grooves defined therein. As described in paragraph 0043 of JP-A 2005-48890, a working fluid is fed into a gap defined between the seal plate 14 and the upper end surface of the inner sleeve 9, so that a thrust bearing 18b is defined therein.
In the conventional motor disclosed in JP-A 2005-48890, an outside surface of the seal plate 14 is a substantially conical surface arranged to become gradually closer to a central axis of the shaft 3 with increasing axial height. Accordingly, an upper portion of the seal plate 14 has a smaller thickness than that of a lower portion of the seal plate 14, and hence, the upper portion of the seal plate 14 has a lower rigidity than that of the lower portion of the seal plate 14. Therefore, when the seal plate 14 is press fitted to the shaft 3, the upper portion of the seal plate 14 will experience a greater deformation than the lower portion of the seal plate 14. This may lead to a reduction in the perpendicularity of a lower surface of the seal plate 14 with respect to the shaft 3, which in turn may lead to a failure to achieve a desired level of dynamic pressure in the thrust bearing 18b, which is defined between the lower surface of the seal plate 14 and the upper end surface of the inner sleeve 9.